Neuroblastomas (NB) are pediatric malignancies with heterogenous phenotypes, ranging from spontaneously regressing to highly aggressive, incurable tumors. Although NB is considered a genetic disease, its etiology and heterogeneity cannot be explained solely by genetic aberrations. NB arises due to defects in sympathetic neuron (SN) differentiation occurring during fetal development. Strikingly, despite the presence of genetic aberrations, NB cells undergo neuronal differentiation when subjected to proper factors. Thus, differentiation defects seem to be independent of genetic changes and can arise due to abnormalities in fetal environment. Strikingly, the two factors promoting de-differentiation of NB cells, hypoxia and glucocorticoids, are elevated in the fetus during maternal stress, suggesting a role for prenatal stress in NB development. Moreover, both hypoxia and glucocorticoids up-regulate neuropeptide Y (NPY), the sympathetic neurotransmitter acting as a mitogenic and survival factor for NB. In line with this, we have found that corticosterone treatment, as well as subjecting pregnant mice to stress associated with experimental procedures alone, increased tumorigenicity in their hemizygous TH-MYCN offspring from 32 to 64%. The role of maternal stress during pregnancy in promoting NB development is further supported by epidemiological data. Increased frequency of NB has been associated with low and high birth weights. Low birth weight is a common sign of prenatal stress, while high birth weight is most often caused by gestational diabetes. Notably both of these conditions are associated with elevated fetal cortisol. Hence, we hypothesize that prenatal stress leads to the blocking of SN differentiation and accumulation of neuroblasts by changing the fetal microenvironment. This, in turn, promotes NB formation by augmenting effects of pre-existing mutations and facilitating accumulation of further genetic changes. To test our hypothesis, we will 1) Determine the effect of prenatal stress on NB development; 2) Identify candidate mediators of its actions and 3) Elucidate the mechanism of this tumorigenic effect. To test the effect of prenatal stress on NB development we will use TH-MYCN mice, which spontaneously develop NB. The mothers during pregnancy will be chronically stressed and tumor frequencies will be compared between their prenatally-stressed and control hemizygous offspring. To identify mediators of the stress effect, we will determine stress-induced changes in concentrations of known stress mediators and overall metabolic alterations in mothers and their offspring. Then, we will test the impact of the candidate stress mediators on SN differentiation, as well as proliferation and survival of neuroblasts and NB cells from prenatally stressed and control animals. We will also determine stress-induced phenotypic and molecular changes in tumors. To our knowledge, this is the first study addressing the role for prenatal stress in NB development. If successful, our research will identify novel pathways involved in NB etiology, which in turn may open new therapeutic opportunities and pioneer preventative approaches for NB - both pharmacological and behavioral.